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Old Jun 10, 2012, 12:00 AM
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Joined Jun 2012
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What C rating batts should I get for a 40A speed controller?

Hey guys, newbie question here.

I'm about to buy a Bixler ARF kit, and I could use some advice about what batteries to get. I'm planning to get an esc and batteries that will allow me to upgrade to a faster motor later down the track.

So I'm looking at the Turnigy Plush 40A speed controller, and 2200mAh batteries (as they are high capacity without being too heavy).

The problem is I'm stuck trying to work out what C rating batteries will allow me to take full advantage of the 40A esc later on when I upgrade the motor. Whatever motor I end up with I'd like it to draw a maximum of 10% below what the esc is rated for, so we're looking at a maximum of ~49.5A burst current and ~36A cont current.

So if 20C batteries will give me a cont current of 44A, then there should be no advantage to getting batteries rated higher than 20C... correct?

Or is it more a case of as the batteries discharge they will not provide as many amps, so perhaps it would be better to get 25C or 30C batteries (or even 35C? 40C?)..


Thanks in advance!
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Old Jun 10, 2012, 12:18 AM
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United States, AK, Fairbanks
Joined Aug 2009
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C ratings are a dark art. There's no real solid way to measure them, so they're more of a guess by the manufacturer... So you can do the math there as far as whether the manufacturer is gonna guess up or down

It takes a hell of a power system to really NEED a pack capable of more than about 20C. Think of it this way: At a 10C draw, a battery is drained completely flat in six minutes. Allowing for a landing run or two and keeping 20% capacity minimum remaining in the pack, you're at around five minutes of flight at a mere 10C draw.

The advantage of a higher C rating is that the pack's lower internal resistance allows the pack to hold higher voltage under load. When you draw current from the battery, its voltage will drop temporarily, often to levels far below nominal voltage. Now, a battery with high internal resistance will experience a big drop in voltage. A weak 3S pack might drop from its fully-charged 12.6V to, say, 9.9V, whereas a real beefy one might only drop to 11.5V at the same current. This little bump in voltage makes a big difference in RPMs and total power.

Packs with higher C ratings tend to be physically bigger and heavier, and price goes up as well. I'd say it's probably just not worth buying more than a 25C or 30C pack at most. The costs begin to outweigh the benefits pretty quick after that.
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Old Jun 10, 2012, 12:42 AM
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Thanks very much, that makes a whole lot more sense now. Cheers
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Old Jun 10, 2012, 12:46 AM
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Hervey Bay, Sunny Qld, Australia
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Quote:
Originally Posted by vonbraun_ View Post

So if 20C batteries will give me a cont current of 44A, then there should be no advantage to getting batteries rated higher than 20C... correct?

Or is it more a case of as the batteries discharge they will not provide as many amps, so perhaps it would be better to get 25C or 30C batteries (or even 35C? 40C?)..


Thanks in advance!
No .........

Most don't mess about with 20C batteries anymore .... I don't . All my lipos nowadays are 40C +. Lower internal resistance means less voltage drop at full throttle and generally similar amperage = more power.

You can DEFINITELY feel the difference flying with higher C rated batteries.
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Old Jun 10, 2012, 02:25 AM
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Okay cheers, will avoid 20C
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Old Jun 10, 2012, 05:45 AM
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Blackpool, Great Britain (UK)
Joined Dec 2003
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Compare two batteries. A 2Ah, 20C and a 2Ah,40C.

The 20C batt is claimed to be OK for a continuous full discharge at 40A [2A x 20].

The 40C batt is claimed to be OK for a continuous full discharge at 80A [2A x 40].

What does "OK" mean? It means that the batt won't get red hot. It won't explode.

But most flyers know from experience that to get good service [ie. many charge-discharge cycles], it pays to limit continuous discharge to about half the suppliers' claims.

If you discharge the batts at only a small fraction of their capability [say at 10A] you should see no difference between them.
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